We had already reported of Siri running on iPhone4 and iPod touch 4G.For some reasons the trick was not unveiled.But now a working trick is out for running Siri on Jailbreaked iPhone 4 and iPod touch 4G.To have Siri work for you, you will need to install a Cydia App named H1Siri.This will completely Port Siri to your device.Just follow the steps below.
Step 1: On your jailbroken device , tap and launch Cydia.
Step 2: Go to Manage -> Sources -> Edit -> Add and add the following repository:
http://www.cydia.be if it doesn’t work for some reasons, try using http://cydia.myrepospace.com/H1Siri/
Once the repository has been added click “Return to Cydia”.
Step 3: Then tap on the repository “cydia.be” or click on search and look for “H1Siri“, once found just tap and confirm the installation.
Step 4: After the installation you will need to reboot your device.
Step 6: Now exit the Settings.app and press and hold the home button and you should see a popup of the actual Siri voice assistant. Just tap and speak whatever you like.
Note: You may not be able to connect to Apple Server and it will give you an error saying “Sorry, I’m having trouble connecting to the network”. You can fix this error by restarting your device couple of times (in this case it worked in two reboots).
The below is how Siri Would look and feel once it is ported to iPhone 4.It Works smooth on Both iPhone4 and iPod touch 4G.
Some have reported that Camera App doesn’t work after installing H1Siri.To fix your camera-recording issues after installing H1Siri, download this file and save it on Desktop. Next step is to SSH your device, if you are using Windows you can use WinSCP and if you’re using Mac OS X you can use Cyberduck, however, we have used iPhone Explorer for the demonstration.
Once you’re logged in navigate to Root/System/Library/Caches/com.apple.dyld, you should now see a file named “dyld_shared_cache_armv7“. Either remove it or replace it with the file you have just downloaded and saved on your Desktop. Now do a reboot and try shooting HD video recording.
If you face any difficulty in installing or found some problem you can try posing your question on here.Try the App on your own Risk!![Source]